I didn't care much for the movie either. The 
manga however is wonderful, thus getting cels 
from X the movie would be worth it for me because 
I love the characters.  One day I may get a cel 
of Kamui, but I HAVE to find one of Subaru.  It's 
been my mission ever since I started really 
collecting cels.  I missed a cel of him once 
(despite the several emails I received from nice 
people trying to get me to head to Taro's 
site...damn the need for sleep).  Hopefully 
another will come around.

But to your question, the cels aren't that rare 
as compared to other series/movies, but they are 
expensive (probably do to Clamp popularity and 
detailed artwork).  However, if you're looking 
for specific characters, it could get annoying to 
find the ones that don't show up much.  Kotori 
shouldn't be too bad.  There are serveral cels of 
her already up on a few sites. In fact Yann has 
one for sale that I really like (but it's $570!).

Have fun finding what you want!

Sumeragi